In general, blood pressure can be measured invasively or non-invasively.
A typical example of the invasive method is directly measuring the pressure of the peripheral artery by inserting a catheter. However, this method involves the risk of arterial bleeding and is improper for the frequent measurement for physical checkup.
A typical example of the non-invasive method is one using a mercury manometer. Such a blood pressure measurement using a mercury manometer is performed as follows. The measurement part is pressed and the pulse is sensed with a stethoscope or fingers, while slowly releasing the pressure. And then the blood pressure can be measured from the height of the column of mercury corresponding to the pulse starting point and pulse end point.
Another non-invasive method is the oscillometric method. In the oscillometric method, a cuff is placed around the upper arm or wrist and inflated until the artery is completely occluded. And then slowly releasing the pressure in the cuff, the values of the pressure oscillation in the cuff is sensed by the pressure sensor and recorded to measure the blood pressure.
In the blood pressure measurement by the non-invasive method, the mean arterial pressure can be calculated by the following Math FIG. 3:MAP=DBP+(SBP−DBP)/3  [Math Figure 3]
where, MAP is mean arterial pressure, DBP is diastolic blood pressure, SBP is systolic blood pressure, and (SBP−DBP) is pulse pressure (PP).
However, the blood pressure measurement using the non-invasive method is restricted in that a continuous measurement is impossible since the cuff needs to be inflated and deflated.
Recently, a blood pressure measurement apparatus which enables a continuous non-invasive blood pressure measurement was developed.
However, the apparatus is complicated because it measures a blood pressure using an electrocardiogram (ECG) and a photoplethysmograph (PPG). Further, because the blood pressure has to be measured at various parts of the body, the patients may feel unpleasant. In addition, the accuracy of the blood pressure measurement tends to be dependent upon the precision of the two devices.